1. Field of the Invention
The present invention relates to a flat panel display device, and more particularly, to an organic electro-luminescent device having a polymer emission layer (EML) and a method for fabricating the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for a uniform luminescence by forming the polymer emission layer by blending a block copolymer and an emission material.
2. Discussion of the Related Art
Recently, with the tendency for a large-sized display, there is an increasing demand for flat-panel display devices occupying less space. One of the flat-panel display devices is an organic electro-luminescent device (hereinafter, referred to as an “organic EL device”), and the technology for the electro-luminescent device has been rapidly developed. Several prototype products have already been demonstrated in the market.
The organic EL device includes an anode formed of a transparent material, such as indium tin oxide (ITO), a cathode formed of a metal (Ca, Li, Al:Li, Mg:Ag, and so on) having a low work function, and a thin organic layer formed between the anode and the cathode. If a forward voltage is applied to such an organic EL device, holes and electrons are injected from the anode and the cathode, respectively. The injected holes and electrons are combined with each other to form excitons. The exciton causes a radiative recombination, which is called the electro-luminescence phenomenon.
Herein, materials for the thin organic layer can be classified into a low molecular material or a high molecular material (i.e., a polymer). In the case of the low molecular material, the thin organic layer is formed on a substrate using a vapor deposition method. In the case of the polymer, it is formed on a substrate using a spin coating method. In order to operate the device at a low voltage, the thin organic layer is formed at a very thin thickness of about 1000 Å. The thin organic layer should be uniform and should not have defects, such as a pinhole.
Also, the thin organic layer is formed of a single material. However, it is generally formed as a multi-layer structure of several organic materials.
The reason for forming the organic device in a multi-layer structure is that the holes and the electrons can be effectively transported to an organic emission layer (EML) when using a hole transport layer (HTL) and an electron transport layer (ETL), since the mobility of the holes is greatly different from that of the electrons in the organic material. If a hole density and an electron density are balanced in the organic emission layer (EML), luminous efficiency is increased.
Additionally, in some cases, an energy barrier of the hole injection can be lowered if a hole injection layer (HIL) formed of a material, such as conductive polymer, Cu—PC or the like, is additionally formed between the anode and the hole transport layer. The energy barrier of the electron injection can be also lowered if a buffer layer (i.e., an electron injection layer), such as LiF layer having a thickness of about 5 to 10 Å, is additionally formed between the cathode and the electron transport layer, such that luminous efficiency is enhanced and a driving voltage is reduced.
If the thin organic layer is formed of a high molecular material (i.e., polymer), the hole injection layer (HIL) and the hole transport layer (HTL) are formed as a single layer and the electron transport layer (ETL) and the electron injection layer (EIL) are not formed.
In the organic EL device, an organic material for the thin organic layer inserted between both electrodes has an advantage in that various types of materials can be easily synthesized because of the simple synthesis path and the color tuning. The organic material is classified into a low molecular material and a high molecular material.
When the thin organic layer is formed of a low molecular material, a driving voltage is lowered and a thin layer having a thickness of about 100 nanometers (nm) is obtained. Also, it is possible to obtain a high resolution and reproduce a natural color. Meanwhile, when the thin organic layer is formed of a high molecular material, a thermal stability, a low driving voltage, and a flexible characteristic can be obtained, and a large-sized device can be manufactured at a low cost. Also, a polymer chain of one dimension is arranged to emit polarized light and an on-off speed is fast.
Thus, depending on organic materials, the organic EL devices can be classified into a low molecular organic EL device using a low molecular material, a high molecular organic EL device using a high molecular material, and a compound organic EL device using both of the low molecular material and the high molecular material. In general, each of these organic EL devices is formed of a multi-layer structure.
FIG. 1 is a cross-sectional view of an organic electro-luminescent device according to the related art.
Referring to FIG. 1, the organic EL device according to the related art includes a substrate 1, a first electrode 2, a hole injection layer 3, a hole transport layer 4, an organic emission layer 6, an electron transport layer 7, an electron injection layer 8, and a second electrode 2.
Here, the second electrode 9 (i.e., cathode) is formed of a metal such as Ca, Mg, Al or the like having a low work function, and lowers the barrier formed between the second electrode 9 and the organic emission layer 6 to obtain a high current density in the electron injection. By obtaining a high current density, it is possible to increase a luminous efficiency of the EL device.
Meanwhile, the first electrode 2 (i.e., anode) is for the hole injection and is formed of a transparent metal oxide having a high work function, so that the emitting light is outputted outside the EL device. Indium tin oxide (ITO) is the most widely used hole injection electrode and has a thickness of approximately 30 nanometers (nm).
Also, the organic emission layer 6 is a material emitting light when holes and electrons injected respectively from the first electrode 2 and the second electrode 9 are combined to form excitons and the formed excitons drop to the ground state. The organic emission layer 6 is formed of a low molecular organic material, such as Alq3, Anthracene or the like, or a high molecular organic material, such as PPV (poly(p-phenylenevinylene)), PT (polythiophene) or their derivatives.
In addition, the hole injection layer 3, the hole transport layer 4, the electron transport layer 7, and the electron injection layer 8 are interposed between the first electrode 2 and the organic emission layer 6 and between the second electrode 9 and the organic emission layer 6 so as to enhance the mobility of holes and electrons, respectively. The respective layers are formed of a low molecular organic material or a high molecular organic material, and their quantization efficiency is enhanced by a combination of the transport layers. The driving voltage of the respective layers is lowered by two steps of injection passing through the transport layer without a direct injection of carriers (electrons or holes). When the electrons and holes injected into the organic emission layer 6 are moved to the opposite electrode through the organic emission layer 6, they are blocked by the transport layer of the opposite side to control the recombination, thereby enhancing the luminous efficiency.
FIG. 1 is a schematic view for the structure of a low molecular organic EL device according to the related art and for a high molecular organic EL device, the hole transport layer 4, the electron transport layer 7, and the electron injection layer 8 are not formed therein.
FIGS. 2A to 2C are energy diagrams of the organic electro-luminescent device according to the related art.
The luminescent principle of the related art organic EL device will be described with reference to FIGS. 2A to 2C. In the drawings, an electron is denoted by the symbol ‘−’, a hole is denoted by the symbol of ‘+’, and a movement of the electron or hole is denoted by an arrow. Also, φA and φC are indicative of work functions of the first electrode and the second electrode, respectively. EA and IP are electron affinity and ionization potential, respectively. HOMO and LOMO are indicative of the highest occupied molecular orbital (valence band) and the lowest unoccupied molecular orbital (conduction band), respectively.
First, as shown in FIG. 2A, if a potential (VCA) is not applied between the first electrode 2 and the second electrode 9, the hole injection layer 3, the hole transport layer 4, the organic emission layer 6, and the electron transport layer 7 are in the thermodynamic equilibrium, so that the Fermi levels of the respective layers coincide with one another.
However, if the potential (VCA) is applied between the first electrode 2 and the second electrode 9, holes are gradually injected into the HOMO of the hole injection layer 3 from the first electrode 2 and electrons are injected into the LUMO of the electron transport layer 7 from the second electrode 9 as shown in FIG. 2B. If the applied voltage (VCA) is lower than a driving voltage or a turn-on voltage (Vonset), holes or electrons are not moved to the organic emission layer 6 and accordingly the electro-luminescence does not occur.
As a result, as shown in FIG. 2C, if the applied voltage (VCA) is higher than the driving voltage (Vonset), holes or electrons pass through the hole injection layer 3, the hole transport layer 4, and the electron transport layer 7, and are injected into the organic emission layer 6, so that the holes and the electrons are recombined to cause the electro-luminescence.
FIGS. 2A to 2C are schematic views for the structure of a low molecular organic EL device according to the related art and in case of a high molecular organic EL device, the hole transport layer 4, the electron transport layer 7, and the electron injection layer 8 are not formed therein.
As described above, the organic emission layer is formed of a low molecular organic material or a high molecular organic material. In the high molecular emission layer using a high molecular organic material, a single high molecular emission layer (i.e., high molecular emission layer of one high molecular material) is used. However, in order to prevent an interchain interaction and enhance the performance of the EL device, researches on a blended system in which different kinds of high molecular materials are blended have been actively performed.
In other words, a dilution effect in which two kinds of luminescent materials are used or different kinds of materials are blended to maintain a distance between main luminescent materials is utilized. At this time, the distance between the main luminescent materials is maintained at about 10 micrometers (□).
Also, in patterning the high molecular emission layer using a laser induced thermal imaging (LITI) method, an inert polymer blend system is used to decrease the cohesion between the high molecular emission materials.
In an early research, PMMA (polymethylmethacrylate) has been used as the inert polymer. Recently, PS (polystyrene)-based material that can be miscible with the high molecular emission material is used as the inert polymer since a phase separation is generated between the PMMA and the polymer.
However, the forming method of the high molecular emission layer using the inert polymer has limitations in that the uniformity of the material used as an inert polymer and the uniform phase spreading between the high molecular emission layers blended with the inert polymer are not guaranteed.